The present invention relates to digital integrators, and more particularly to the automatic scaling of input samples to a digital feedback integrator in accordance with a change in the number of samples to be integrated.
Digital integrators of the feedback type are widely utilized in many fields of electronics to implement an integrating function and more specifically, to implement a single-pole digital filter. They are especially useful in the area of signal processing for integrating or auto-correlating several signal returns in order to enhance a signal relative to a noise background. In this regard, it is often desirable to change the number of samples integrated. For example, in one instance it may be desirable to integrate 1024 samples while in another instance 8 might be more appropriate.
A feedback integrator operates by adding to the input signal a weighted value of past integrator output. The sum becomes the new integrator output and is stored in the integrator's register. In this process, the weighting factor in effect determines the integrator's memory time, or period of integration.
Unfortunately, utilization of digital integrators has not been without its problems. One that has been particularly bothersome is the effect on an analog display which displays the result of the digital integration. The display operates satisfactorily when the feedback integrator is integrating the largest number of samples which the integrator is designed to process. When the integrator is processing the maximum number of maximum amplitude signals, then after all the samples have been processed, the integrator's register is filled to its capacity, representing a maximum magnitude integration result. A digital-to-analog converter then translates the number stored in the register into a full screen deflection voltage.
But when the sample size selected for integration is smaller than the maximum designed size, a problem of reduced output dynamic range occurs -- a problem which becomes progressively worse as the number of samples selected for integration become smaller.
The dynamic range problem arises in that the register, designed to accumulate a maximum number of samples, remains fairly empty when only a small number of samples are integrated. For example, a feedback integrator may be designed to process 1024 samples of a signal quantized into 16 levels (representing zero through fifteen). The sixteen levels may be represented mathematically and electrically by four binary digits, or bits. If each of the 1024 samples was of its maximum value of 15, then a register to store them all would need storage space equivalent to the representation of the quantity 1024 .times. 15, or 15,360. This could be stored in a register containing 14 binary bits (providing a maximum representation of 16,384).
Now, if the operation of the feedback integrator were changed so as to integrate only 8 samples, the register would fill to a much lesser extent. For example, 8 maximum-level signals would occupy the binary equivalent of only 8 .times. 15 = 120. This can be represented and stored in a binary register containing only 7 binary bits.
Regarding digital processing devices of this nature, the most significant digit stored in the binary register is generally found in the left-most position. Conversely, the least significant digit is then generally to be found in the right-most register position.
The digital-to-analog device which converts the binary number stored in the register to an analog display signal attributes half of the total maximum display signal magnitude to the most significant position (binary digit) and half of the remaining magnitude to the remaining bits. In this manner, the display converter assigns less and less weight to digits located respectively in the less significant register bit positions. Because of this, the magnitude of the signal to the display system is primarily derived from the most significant register bit positions.
8 an integration of 1024 maximum-valued samples might, for example, be converted to read 15 volts for a full scale display reading, an integration of only eight full scale samples would yield only 120/15360 or 0.013 of the full scale of 15 volts, resulting in a voltage of 0.198 volts. Clearly this value is very much less than the 15 volt signal needed for full scale deflection of the display.
As the number of samples to be integrated is reduced, the full scale output voltage to the display is reduced in like proportion. In order to compensate for the lowered output voltage associated with the reduction in number of samples so that full scale deflection is maintained, prior techniques have gone in two major directions. The first method is easily implemented and inexpensive. It is to simply increase the sensitivity of the display system so that the much lower analog display voltage will cause a full scale deflection of the display. Unfortunately this causes an undesirable condition in which noise-signals in the system become quite evident in the display presentation (due to the additional amplification). These displayed noise signals can interfere with detection of a true signal by masking it.
The second method is moderately effective but expensive. Switch connections are implemented between the shift-register (storing the result of the integration) and the output digital-to-analog converter. The switching is arranged so that the most-significant-bit position of the converter is connected to the shift-register position containing the most-significant-bit that results from the integration of the smaller sample size. For a typical digital-to-analog display signal converter having 20 binary-bit input positions, an ability to typically switch 20 positions among several shift-register output terminals is required. Switching of this nature tends to be costly and unreliable.
The present invention accomplishes scaling of the samples to be integrated so that an input signal having a maximum value will result in a maximum value of display voltage from the digital-to-analog converter, independent of the number of samples selected for integration.